Worldwide efforts are underway to improve the quality of video signal production, transmission, and reproduction because a great deal of commercial importance is being predicted for improved quality video systems. These efforts involve, at least in part, increasing the resolution with which are converted into representative electrical signals by increasing the spatial and temporal sampling rates that are used to convert video images into electrical signals. This increase in resolution consequently means that more data about images must be produced, processed, and transmitted in a given period of time.
Video images such as those images in the field of view of a television camera are scanned at a predetermined rate and converted into a series of electrical signals, each electrical signal representing a characteristic of a predetermined region of the image generally referred to as a picture element, pel, or pixel. A plurality of the picture elements taken together at a predetermined instant of time form what amounts to a still picture representing the nature of the image at the predetermined instant of time. Increasing the quality of video signals produced in this manner involves, at least in part, the use of larger number of smaller-size picture elements to represent a given image frame and the production of a large number of image frames per unit time. For example, the CCIR-601 recommendation specifies the number of picture elements in a frame to be 720 horizontal picture elements.times.486 vertical picture elements (U.S. and Japan) or 576 vertical picture elements (Europe). Thirty or 25 interlaced pictures are produced each second. In high definition television (HDTV) projects, it has been proposed to have about 700-1000 horizontal lines each having 1200-2000 picture elements. These HDTV efforts contemplate production of 25 or 30 interlaced pictures per second or 60 or 50 non-interlaced pictures per second.
As the number of picture elements for each video frame and the rate at which frames are produced increases, these is an increasing amount of video data which must be produced, transmitted, and received in a given period of time. It would be advantageous if video signals produced by these systems could be compressed so that a smaller amount of data could be generated which would still contain enough information so that higher quality video images could be reproduced.
A number of data compression schemes have been proposed which attempt to transmit higher quality video images using the same numbers of bits and the same bit rates used for lower quality images. One such scheme involves an encoder which receives digital video signals representing the characteristics of a sequence of picture elements. The encoder transforms blocks of such video signals into blocks of transform coefficients relating to the spatial frequency components in the areas of the image represented by the blocks of picture elements. The blocks of frequency coefficients are then quantized and scanned according to some predetermined sequence. The quantized frequency coefficients are then sent in the order defined by the scanning sequence to a variable word length coder then encodes the quantized frequency coefficients and then transmits the encoded quantized frequency coefficients. It has been found that less bits need to be sent when these encoded quantized frequency coefficients are sent instead of pixel data bits.
Another data compression scheme which has been proposed involves estimating the characteristics of a segment of video signal and subtracting the estimate from the actual segment of video signal to produce an estimate error signal which is then coded and transmitted instead of the actual segment of video signal. Again, it has been found that a lesser number of bits need to be transmitted when estimation error signals are transmitted in place of pixel data signals.
Yet another technique of compressing video data involves the production and transmission of data representing motion vectors calculated in light of a current segment of video signal and a prior segment of video signal instead of transmission of pixel data. These motion vectors may be used to provide motion compensation for producing more accurate estimates of a video signal and smaller estimate error signals, which thereby reduces the number of bits which must be used to transmit video signals.
Each of these techniques seeks to send data derived from an actual video signal which can be used by a decoder in a receiver of the video signals to reconstruct the actual video signal from a limited subset of the data defined the actual video signal. The actual number of bits which must be transmitted in these situations is less than the number of bits needed to define each picture element in the video signal and, thus, higher resolution video signals may be transmitted at the same bit rate. Although each of these techniques is to a certain degree successful in achieving suitable compression of video data without undue loss of information, there are significant areas whereby the encoding of video data may be improved so that the number of bits which must be transmitted is reduced and an accurate reconstruction may be made by a video decoder.